private Rect _yourGroupRect;
private Matrix4x4 _matrixBackup;
public float Rotation = 0; // integer (in degrees)
public Vector2 CenterOfRotation; // most probably you want to rotate around the center, so set this to (_yourGroupRect.width/2, _yourGroupRect.height/2)
public void Draw() {
_matrixBackup = GUI.matrix;
GUIUtility.RotateAroundPivot(Rotation, CenterOfRotation);
GUI.BeginGroup(_yourGroupRect);
// draw your GUI elements here
GUI.EndGroup();
GUI.matrix = _matrixBackup;
}